This crate defines an opinionated strategy for error handling in Rust,
built on the following principles:

No error should ever be discarded. This library primarily
makes it easy to "chain" errors with the chain_err method.

Introducing new errors is trivial. Simple errors can be introduced
at the error site with just a string.

Handling errors is possible with pattern matching.

Conversions between error types are done in an automatic and
consistent way - From conversion behavior is never specified
explicitly.

Errors implement Send.

Errors carry backtraces.

Similar to other libraries like error-type and quick-error, this
library defines a macro, error_chain! that declares the types
and implementation boilerplate necessary for fulfilling a
particular error-hadling strategy. Most importantly it defines
a custom error type (called Error by convention) and the From
conversions that let the try! macro and ? operator work.

This library differs in a few ways from previous error libs:

Instead of defining the custom Error type as an enum, it is a
struct containing an ErrorKind (which defines the
description and display methods for the error), an opaque,
optional, boxed std::error::Error + Send + 'static object
(which defines the cause, and establishes the links in the
error chain), and a Backtrace.

The macro additionally defines a trait, by convention called
ChainErr, that defines a chain_err method. This method
on all std::error::Error + Send + 'static types extends
the error chain by boxing the current error into an opaque
object and putting it inside a new concrete error.

It provides automatic From conversions between other error types
defined by the error_chain! that preserve type information,
and facilitate seamless error composition and matching of composed
errors.

It provides automatic From conversions between any other error
type that hides the type of the other error in the cause box.

It collects a single backtrace at the earliest opportunity and
propagates it down the stack through From and ChainErr
conversions.

To accomplish its goals it makes some tradeoffs:

The split between the Error and ErrorKind types can make it
slightly more cumbersome to instantiate new (unchained) errors
errors, requiring an Into or From conversion; as well as
slightly more cumbersome to match on errors with another layer
of types to match.

Declaring error types

Generally, you define one family of error types per crate, though
it's also perfectly fine to define error types on a finer-grained
basis, such as per module.

Assuming you are using crate-level error types, typically you will
define an errors module and inside it call error_chain!:

error_chain! {
// The type defined for this error. These are the conventional
// and recommended names, but they can be arbitrarily chosen.
types {
Error, ErrorKind, ChainErr, Result;
}
// Automatic conversions between this error chain and other
// error chains. In this case, it will e.g. generate an
// `ErrorKind` variant called `Dist` which in turn contains
// the `rustup_dist::ErrorKind`, with conversions from
// `rustup_dist::Error`.
//
// This section can be empty.
links {
rustup_dist::Error, rustup_dist::ErrorKind, Dist;
rustup_utils::Error, rustup_utils::ErrorKind, Utils;
}
// Automatic conversions between this error chain and other
// error types not defined by the `error_chain!`. These will be
// boxed as the error cause and wrapped in a new error with,
// in this case, the `ErrorKind::Temp` variant.
//
// This section can be empty.
foreign_links {
temp::Error, Temp,
"temporary file error";
}
// Define additional `ErrorKind` variants. The syntax here is
// the same as `quick_error!`, but the `from()` and `cause()`
// syntax is not supported.
errors {
InvalidToolchainName(t: String) {
description("invalid toolchain name")
display("invalid toolchain name: '{}'", t)
}
}
}

This populates the the module with a number of definitions,
the most important of which are the Error type
and the ErrorKind type. They look something like the
following:

This is the basic error structure. You can see that ErrorKind
has been populated in a variety of ways. All ErrorKinds get a
Msg variant for basic errors. When strings are converted to
ErrorKinds they become ErrorKind::Msg. The "links" defined in
the macro are expanded to Dist and Utils variants, and the
"foreign links" to the Temp variant.

Both types come with a variety of From conversiaos as well:
Error can be created from ErrorKind, from &str and String,
and from the "link" and "foreign_link" error types. ErrorKind
can be created from the corresponding ErrorKinds of the link
types, as wall as from &str and String.

into() and From::from are used heavily to massage types into
the right shape. Which one to use in any specific case depends on
the influence of type inference, but there are some patterns that
arise frequently.

Returning new errors

Introducing new error chains, with a string message:

fn foo() -> Result<()> {
Err("foo error!".into())
}

Introducing new error chains, with an ErrorKind:

fn foo() -> Result<()> {
Err(ErrorKind::FooError.into())
}

Note that the return type is is the typedef Result, which is
defined by the macro as pub type Result<T> = ::std::result::Result<T, Error>. Note that in both cases
.into() is called to convert a type into the Error type: both
strings and ErrorKind have From conversions to turn them into
Error.

When the error is emitted inside a try! macro or behind the
? operator, then the explicit conversion isn't needed, since
the behavior of try! will automatically convert Err(ErrorKind)
to Err(Error). So the below is equivalent to the previous:

Chaining errors

To extend the error chain:

use errors::ChainErr;
try!(do_something().chain_err(|| "something went wrong"));

chain_err can be called on any Result type where the contained
error type implements std::error::Error + Send + 'static. If
the Result is an Err then chain_err evaluates the closure,
which returns some type that can be converted to ErrorKind,
boxes the original error to store as the cause, then returns a new
error containing the original error.

Foreign links

Errors that do not conform to the same conventions as this library
can still be included in the error chain. They are considered "foreign
errors", and are declared using the foreign_links block of the
error_chain! macro. Errors are automatically created from
foreign errors by the try! macro.

Foreign links and regular links have one crucial difference:
From conversions for regular links do not introduce a new error
into the error chain, while conversions for foreign links always
introduce a new error into the error chain. So for the example
above all errors deriving from the temp::Error type will be
presented to the user as a new ErrorKind::Temp variant, and the
cause will be the original temp::Error error. In contrast, when
rustup_utils::Error is converted to Error the two ErrorKinds
are converted between each other to create a new Error but the
old error is discarded; there is no "cause" created from the
original error.

Backtraces

The earliest non-foreign error to be generated creates a single
backtrace, which is passed through all From conversions and
chain_err invocations of compatible types. To read the backtrace
just call the backtrace() method.